As I understand your question, you feel that the speaker in saying
There was, chewing gum..no not chewing. Popping.
is making a distinction where none exists: chewing and popping are in your view the same or very similar and you seek a term for this rhetorical device.
As an aside, in my view the distinction between Chewing and Popping is actually valid. I speak from the heart: I have a seat at the football, and every match the chap behind me chews gum, I don't care at all. BUT he pops it right behind my head, this is amazingly annoying! Being British of course, I say nothing :-) So in this case, I don't think this is actually a rhetorical device, it's just clarifying that some uses of gum are more annoying than others.
However, can imagine similar scenarios where such a rhetorical device is being used, how would we describe such an elevation of a very minor, or immaterial, distinction?
In business meetings I've heard the phrase "A distinction without a difference", but that doesn't fit this more rhetorical situation. Another phrase is "making a mountain out of a molehill" but again this doesn't quite fit as this implies exaggerating something small but real. In this context I'd consider using
Straining at a gnat
leaving unsaid the
while swallowing a camel
my implication being that the speaker is deeply offended by the trivial popping and in the wider context sees no problem in shooting someone.
Another description, which I do think fits:
making something out of nothing
You are seeking a concise term for this kind of rhetorical device, and I've failed to find a precise one. The effect is of putting two related terms side-by-side so it seems to a form of
If done in a knowing sort of a way we could say that we are using an
We could say that the speaker thinks they are using
that is producing a climax, "not just chewing, POPPING.", but in our eyes failed to do so, and as this is done in an unknowing way, where we the listener somewhat sneer at this attempt we get a